The grout is soft & powdery [Photo 1]. Usually the grout readily fails when scratched.
Technical reasons for soft & powdery grout
Morgan advises that the grout was not properly mixed and that soft & powdery grout joints are an installation fault created by the tradesman. Grouts that are soft and powdery usually suffer from the addition of too much water to the original mix. Too much water creates a low density grout that cannot cure correctly and becomes inherently soft. Another reason for a soft & powdery grout is due to water loss from the grout before the grout has had an opportunity to cure. This can happen due to the absorption of grout water into the tile and substrate or due to water evaporation caused by adverse environmental factors such as strong sunlight, heat or wind.
Recommended procedure for repairing soft & powdery grout
It is important to determine the cause of the soft & powdery grout first.
Grouts that have been mixed with too much water will usually shrink back into the grout joint and support a white efflorescence layer at the surface. Grouts that have been mixed with too much water therefore need to be removed completely and the entire tile installation needs to be re-grouted. Grouts installations that have been dehydrated before curing generally are flush with the tile joints but are weak and friable. These types of faulty installations can usually be hydrated using a damp curing method. Good results can generally be obtained with clean water and a mop & bucket.